7 replies to this topic

[mohamad]

Dear Friends

What is balancing or equalizing line between steam reboiler and steam condensate pot ?

i saw in waste water stripper column that two (about 2inch) lines are connecting top of cond pot to inlet and outlet of reboiler steam side. 

but in another column i saw there was just a line connecting top of pot to reboiler steam side outlet

what is the difference?

[fallah]

mohamad,

 

In general, reboiler balance line is a line connecting the condensate pot to the reboiler inlet in order to pressure equalization between mentioned pot and the reboiler. Such balancing allows the condensate will be easily conducted to the pot from the reboiler.

 

To say about the difference of the two cases you mentioned, it's needed having at least schematic diagrams of those cases...

Edited by fallah, 14 July 2014 - 04:52 AM.

[mohamad]

i used paint to draw a rapid schematic diagram in attached file

Attached Files

•  BL.jpg   26.09KB   117 downloads

[fallah]

mohamad,

 

In the case with two balance lines there might be the possibility of noncondensable gases collection below the pass partition and in addition to one balance line connecting top of the pot to the inlet of the reboiler steam side another balance line has connected top of the pot to the outlet of the reboiler steam side below the pass partition to prevent any pressure unbalancy betweeen inlet/oulet of the reboiler steam side.

 

Of course, in my opinion as long as the balance line will connect top of the pot to below the pass partition of the reboiler steam side, single balance line would be adequate for all applications with or without the possibility of noncondensable gases collection below the pass partition...

[mohamad]

thank you for your reply

but with just one line connecting top of the pot to below the pass partition plate we cannot balance pressure between inlet/outlet of steam side. and there would be pressure drop in steam pass in the reboiler what would effect heat transfer. 

i am not sure but i think a line between pot and steam inlet is enough what would balance pressure in the reboiler and let the condensed water to flow to the pot by gravity. but i have seen these different cases in chemical plants.

 

and for the problem of non-condensable gas how does it help (the balance line)?

[fallah]

mohamad,

 

The main point is that the pressure in below the pass partition baffle and the pressure in the condensate pot to be the same, otherwise the level in the condensate pot might not indicate the level in the channel head.

[Art Montemayor]

mohamad,

 

This is a very basic engineering application that has been discussed numerous times in our Forums.  For some reason, the principles of condensate formation and vapor-liquid separation are not well understood by some members.  Allow me to reinforce what Fallah has so kindly furnished as experienced, correct recommendations:

 

Please refer to the attached Excel workbook for a detail of what I see as mistaken balance line applications in your two installations.  It is obvious to me that both of your existing installations have been done wrong in a “trial and error” method and not with an accurate and correct engineering understanding of what is going on.

 

A balance or equalization line is a small diameter pipe that tries to equalize the pressures existing on the top of two vessels to allow for free, unrestricted draining of liquid from one vessel to another.  In other words, it is an engineering application of using GRAVITY to one’s advantage.  Without this equalization of pressure, it is difficult to ensure that the required draining will be done efficiently.  The principle is as simple as that, and it should be applied it in the simplest manner possible.

 

You are wrong in stating “with just one line connecting top of the pot to below the pass partition plate we cannot balance pressure between inlet/outlet of steam side”.  That is simply not true.  There is no flow established in the vapor balance line; the steam in it is STATIC – it can’t flow because there is no displacement.  By forcing the formed condensate to flow through the tubes due to gravity, you are ensuring that there will be on-going displacement of fluid within the reboiler’s tube bundle.  THAT IS WHAT IS SUPPOSED TO HAPPEN.   Your correct balance line is the one I show in RED in the workbook.

 

You are also wrong is stating “i think a line between pot and steam inlet is enough what would balance pressure in the reboiler and let the condensed water to flow to the pot by gravity”.  While you might obtain condensate flow into the condensate pot, it will also be against a slightly higher pressure (the steam pressure in the steam inlet is higher).  Therefore, you are not ensuring that the condensate level in the pot will be an accurate one.  Fallah is absolutely correct in what he is recommending to you.  Common sense should tell us that is the reason the equalization line is called exactly that – EQUALIZATION.

 

If you expect to accumulate non-condensables in the tube bundle and condensate pot (and I seriously believe you will), then that is another problem that is solved rather easily.  You require a separate non-condensable vent from the balance line or below the tube bundle’s pass partition plate with a vent valve that is opened from time-to-time, accordingly to allow for venting the undesired, accumulated non-condensables.  This is done either manually or automatically.  How often you have to do this depends on your system and how efficiently you operate your steam system.

 

 Reboiler Condensate Balance Line.xlsx   37.35KB   490 downloads

[Bobby Strain]

My personal practice is to use a condensate pot only to keep live steam from passing into the condensate line. Every time I started up a facility that utilized the pot level to manipulate the duty, it never worked. So, I implemented my standard fix, with 100% success. Now, if designers follow this basic principle, where you locate the equalizing line is only a function of the exchanger elevation. To achieve the lowest elevation, what Art shows will achieve this. If elevation is of no consequence, the equalizing line can be connected into the inlet channel, or the inlet steam line, and the pot and exchanger located to assure proper drainage. Consider, too, that many of the folks who license designs, like UOP, have a preference for using a condensate valve with a flow meter between the exchanger and the valve. This provides the maximum steam pressure and saves money by using a much smaller control valve. The elevations are important in this arrangement to assure a good-working flow meter. Always rely on calculations, no matter how simple a system may seem. And try to use the best practice, which might be a combination from several sources.

 

Bobby